1. Field of the Invention
The invention relates to a vehicle four-wheel drive apparatus, and particularly to a four-wheel drive apparatus having a connection/disconnection mechanism that selectively connects or disconnects a power transmission for transmitting power to a propeller shaft of auxiliary drive wheels.
2. Description of Related Art
A vehicle four-wheel drive apparatus for transmitting power of a drive source to main drive wheels and selectively transmitting a portion of the power to auxiliary drive wheels is well known. Further, in the vehicle four-wheel drive apparatus, a four-wheel drive apparatus is proposed, which is provided with a first connection/disconnection mechanism that connects or disconnects a power transmission between a transfer for distributing the power of the drive source to the auxiliary drive wheels and a propeller shaft and a second connection/disconnection mechanism that connects or disconnects a power transmission between the downstream side of the propeller shaft and the auxiliary drive wheels, and to disconnect these connection/disconnection mechanisms during two-wheel driving traveling to eliminate traction of the propeller shaft and the like and improve fuel efficiency. The vehicle described in Japanese Patent Application Publication No. 2013-159300 (JP 2013-159300 A) is an example thereof.
The vehicle described in JP 2013-159300 A is configured to have a first connection/disconnection mechanism (the first connection/disconnection mechanism) that is provided in a power transmission device which is arranged on the front wheel side and that connects or disconnects a power transmission between a first rotation shaft and an output unit, and an axle disconnect that is a second connection/disconnection mechanism (the second connection/disconnection mechanism) and that connects or disconnects a power transmission between a propeller shaft and rear wheels. Here, the first connection/disconnection mechanism that is arranged on the front wheel side is a dog clutch that connects or disconnects the first rotation shaft to or from the output unit by selectively engaging or disengaging teeth that are formed on a connection/disconnection member which is splined with the first rotation shaft and thus is not rotatable relative to it to or from opposite teeth that are formed on the output unit. In addition, as for the first connection/disconnection mechanism, if an electromagnet constituting an actuator is energized, the connection/disconnection member is connected by moving toward one end (an engagement position) in an axial direction, and if the power supply is stopped, the connection/disconnection member is disconnected by moving toward the other end (a disengagement position) in the axial direction by an action force of a return spring. Further, the axle disconnect (the second connection/disconnection mechanism) is configured of a multi-plate clutch that can adjust a torque transmission capacity.
In the vehicle of JP 2013-159300 A, the second connection/disconnection mechanism is configured of the multi-plate clutch. However, when the first connection/disconnection mechanism and the second connection/disconnection mechanism are disconnected, even if a torque of the multi-plate clutch that is the second connection/disconnection mechanism is controlled to zero, a traction torque is generated at the multi-plate clutch and is transmitted to the first connection/disconnection mechanism side via the propeller shaft, and thus becomes a resistive force for moving a sleeve (the connection/disconnection member) in the axial direction when the first connection/disconnection mechanism that is the dog clutch is disconnected. Therefore, in order to ensure a thrust force that is required for switching of the first connection/disconnection mechanism and a responsivity of the switching, the action force that is applied by the return spring to the disengagement side shall be increased. However, as a result of increasing the action force that is applied by the return spring, an electromagnetic force that is required for connecting the first connection/disconnection mechanism becomes large, and thus the electromagnetic solenoid becomes large, resulting in that mountability of the electromagnetic solenoid is degraded. Further, in the first connection/disconnection mechanism of JP 2013-159300 A, even if the electromagnetic solenoid is disconnected when being energized and is connected by the return spring when the power supply is stopped, the electromagnetic solenoid becomes large in order to ensure the responsivity of the switching of the first connection/disconnection mechanism, and thus the mountability of the electromagnetic solenoid is degraded.
Further, when the dog clutch is switched by for example, a motor, a reduction gear is required to obtain the thrust force that is required for the switching, and thus the responsivity of the switching is decreased. Further, when the dog clutch is switched by for example, a hydraulic mechanism, although the thrust force that is required for the switching can be secured, there is a problem that the responsivity of the switching is decreased as the oil viscosity is increased when the oil temperature is low, for example. Moreover, switching of a drive state of a four-wheel drive apparatus is based on switching of connection/disconnection states of the first connection/disconnection mechanism and the second connection/disconnection mechanism, and the responsivity thereof is dependent upon the connection/disconnection mechanism whose operating time that is required for the switching is longer among these connection/disconnection mechanisms. Therefore, if the operating time of any one of the connection/disconnection mechanisms becomes longer, the responsivity of the switching of the drive state of the four-wheel drive is decreased.